Special process steps are necessary to make native starch sufficiently thermoplastically formable. In particular, native starch must be in the destructured form before being thermoplastically formed. For this purpose, in general, native starch having a water content of 5% to 18% by weight is extruded while being exposed to intense shear forces at 100.degree. C. to 200.degree. C. Plasticizers, such as polyhydroxy compounds; destructurizing agents, such as alkali metal salts, aqueous ammonia, urea, and urea derivatives; and mold release agents, such as metal stearates, are added to improve subsequent processing.
It is known that it is easier to obtain thermoplastic compositions from amylose-rich starches. In addition, starch compositions wherein the OH groups of the glucose units are reacted to a small extent (chemically modified), in particular ethoxylated or propoxylated, are easier to thermoplastically process. At the same time, they show improved long-term stability because the tendency of the amylose chains to retrograde is greatly reduced. Starch compositions of this type are described in DE-A 1,470,965; GB 965,349; and U.S. Pat. No. 3,117,014.
Methods in which a starch composition with a high water content is processed under elevated pressure are taught in EP 3 118 240 and EP A 327 505. The shrinkage, which can amount to up to 50%, is an extreme disadvantage in these cases. This main disadvantage is counteracted by addition of a foreign polymer such as polyethylene (PE), polystyrene (PS), and polyacetal, as well as olefinic copolymers such as ethylene acrylic acid (EAA) and ethylene vinyl acetate (EVA).
Further difficulties which occur despite observing the teachings of the state of the art in the preparation and processing of starch compositions, even if chemically modified or having a high amylose content, are substantial. Discoloration is often unavoidable; processing by injection molding is difficult because the unmolding is poor; the injection moldings have surface defects and are prone to adhesion, warping, and shrinkage. If the compositions are processed in an extruder, for example to form films, there is often a high surface roughness and tendency to adhesion.
The object therefore is to overcome the disadvantages of the state of the art by a destructured starch composition which can be prepared and thermoplastically processed straightforwardly and reliably. (Throughout the specification and claims, unless otherwise stated, all parts and percentages are by weight).